A communication system to be mounted on a car is heretofore adopted in which electronic control units (ECUs) for controlling the operation of apparatuses mounted on the car are connected to each other with communication lines connected to each other via a relay connection unit (gateway apparatus), and messages sent and received among the ECUs belonging to different communication lines are relayed by the relay connection unit.
Upon reception of a message from the ECU connected to one communication line, the relay connection unit relays the message to other communication lines to which the message is to be relayed.
In detail, the relay connection unit has a routing map in which the correspondence between an identifier of a received message and the communication line connected to the ECU to which the message is to be relayed is described. Upon receipt of the message, the relay connection unit reads the identifier thereof and determines a communication line, corresponding to the identifier thereof, to which the message is sent with reference to the routing map, thus sending the message to the identified communication line.
As a method of relaying the relay connection unit, a cut-through system is known, as described in Japanese Patent Application Laid-Open No. 9-162917.
As shown in FIG. 8, in the cut-through system, the relay connection unit receives a message from a communication line (receiving-side bus). At a time point tb when the relay connection unit has received an arbitration field (ID) of the message in which a message identifier is written, the relay connection unit determines a communication line (destination bus), corresponding to the message identifier, to which the message is to be relayed with reference to the routing map and starts to send the message to the communication line. Because the relay connection unit starts to send the message to the communication line without receiving the message to the end, the period of time required from the time the message is received till the transmission of the message is started is shorter than that required in a store & forward system in which transmission of the message is started after the message is received to the end. Thus the cut-through system performs relay processing at a high speed.
But in the relay processing of the message executed by the relay connection unit in the cut-through system, as shown in FIG. 9, when the relay connection unit sends a message received from one communication line (bus) which is the receiving side at a transmission speed of 250 kpbs to other communication line (bus) which is the destination at a transmission speed of 500 kbps, the message reception speed is lower than the message transmission speed. Therefore data of a message to be sent is not received, and there arises a problem that a state in which there is no data to be sent is generated.
When the state where there is no data to be sent is generated, the relay connection unit suspends the relay processing. As a result, a problem that an error is generated on the bus arises.    Patent document 1: Japanese Patent Application Laid-Open No. 9-162917